Add an optional delegate method to change the color of dimming background overlay

[Arthraim]

Hi Jared. Since our designer prefers a white overlay, I made a little change based on your great work. Hope the change would be merged into your trunk.

Say I do something like ...

- (UIColor *)colorForBackgroundDimmingOverlayInImageViewer:(JTSImageViewController *)imageViewer {

    return [UIColor redColor];
}

It shows like ...

[honkmaster]

Nice feature, is a merge intended?

[Arthraim]

@honkmaster yes, please :smile: